næ, dependent upon the reciprocal action of parallel waves of light of different velocities, and capable in their different combinations of pro- ducing all the colors of the rainbow, or the absence of color, furnish that dazzling chromatic gamut which Nature emjtloys to paint the humming-bird and the butterfly, those two jewels of the organic world. Another class of phenomena has been called cerulescence by M. G. Pouchet. It is a property which he regards as analogous to fluo- rescence, and as due, in the majority of cases, to stick-sha])ed bodies inclosed in special cells called iridocytes. The blue reflections pre- sented by the scales of most fishes, the blue color of the caruncles of many birds, and the naked parts of some monkeys, the azure tint of the veins of individuals of the white race, the blue of the iris of some persons' eyes, are examples of cerulescence. These phenomena, bow- ever, differ but little from those Avhich give to water having drops of milk suspended in it a bluish color by reflection, or from those which make smoke appear blue when seen upon a black ground. It seems to me that it may be going too far to compare such phenomena with fluorescence.
These causes of coloration may be superposed and combined in a thousand ways. When birds are under the influence of physiological excitations like those of rage or love, the flow of blood contributes to enliven the color of the bare parts to the point of greatly modifying it. The bright, metallic tints of the peacock and the humming-bird are due to phenomena of interference and to the presence of a dark pig- ment combined ; the green tint of the lizard to the association of a yellow pigment and blue-reflecting iridocytes. The Annelids and the Nemertes, of the invertebrates, exhibit the combined effects of three causes of coloration : iridization, produced by the thin cuticle ; the rich pigmentation of the dermis, and frequently, also, in case the in- teguments are transparent, the variable coloration of the sanguineous fluid and of the internal organs.
The intensity of coloration is generally proportioned to the vital activity. As life begins to decline, the pigment retires from the for- mations of the epidermis ; and the hairs on regions which have passed maturity often exhibit a lighter coloring than on the neighboring regions. According to Pruncr-Bey, the intensity of the color of the negro is an indication of his health ; old negroes grow pale as they age. It is well known that pain and depressing moral trials, which are negative facts in life, provoke the retraction of the pigment. On the contrary, everything that tends to accentuate life occasions an enlivening of the intensity of colors, a fact of which T>arwin gives many examples in his " Variation of Animals and Plants." Coloration is strongest in adult animals. Breeders prefer animals rich in i»ig- ment-matter, because they will best resist disease, and most easily accommodate themselves to special svstems of foedinix. The ancients regarded animals having white hair on a black skin as the most vigor-